In an elevator installation, an elevator car and a counterweight are conventionally supported on and interconnected by traction means. The traction means is driven through engagement with a motor-driven traction sheave to move the car and counterweight in opposing directions along the elevator hoistway. The drive unit, consisting of the motor, an associated brake and the traction sheave, is normally located in the upper end of the elevator hoistway or alternatively in a machine room directly above the hoistway.
Safety of the elevator is monitored and governed by means of a safety circuit or chain containing numerous contacts or sensors. Such a system is disclosed in U.S. Pat. No. 7,353,916. Should one of the safety contacts open or one of the safety sensors indicate an unsafe condition during normal operation of the elevator, the controller instructs the drive to perform an emergency stop by immediately de-energizing the motor and applying the brake. The elevator cannot be called back into normal operation until the reason for the break in the safety circuit has been investigated and the relevant safety contact/sensor reset.
Traditionally, steel cables have been used as traction means. More recently, synthetic cables and belt-like traction means comprising steel or aramid cords of relatively small diameter coated in a synthetic material have been developed. An important aspect of these synthetic traction means is the significant increase in the coefficient of friction they exhibit through engagement with the traction sheave as compared to the traditional steel cables. Due to this increase in relative coefficient of friction, when the brake is applied in an emergency stop for an elevator employing synthetic traction means there is an significant increase in the deceleration of the car which severely degrades passenger comfort and could even result in injury to passengers.
GB-A-2153465, U.S. Pat. No. 5,323,878 and U.S. Pat. No. 5,244,060 all describe methods of controlling the movement of an elevator car during an emergency stop wherein the brake is automatically and immediately applied but the degree of the brake force or torque exerted by the brake is dependent on the load of the car. These methods help reduce deceleration of the elevator car during an emergency stop.